Recreational & Family - Offroad tires on a hybrid

I recently bought a Gary Fisher Zebrano that came with road tires, but my brother bought a used Cannondale Scalpel 2000 and wants to go offroading. do you think it's a good idea to buy offroad tires and put them on the Zebrano and try some offroading? As far as I know the Gary Fisher Zebrano uses a Hardtail frame and road wheels.

To save us all looking up the website, could you tell us what size your current tyres are. It is marked on the sidewall.

Bontrager Invert Aramid HD 700x35c

I was thinking about changing out the old wheels and tires with 26" mtb set?

you cant do that, the brake pads wont line up. and its a waste of money. the zebrano has the following problems, which limit it to dirt roads, and gravel paths. it would be dangerous to take it mtn biking per se. the center of gravity is too high, making the bike unstable in off road manuevering situations. the fork would revolt, and be a detriment to handling. it has a cheap adjustable stem, which is an off road liability. it has shifters which wouldnt shifter under the higher torque of mtn biking. the list goes on. keep it on casual rides, save up for a decent mtn bike, ie save about 700 bucks for a started bike

You can ride along trails with your bike and you could probably tackle some quite technical sections but you should avoid doing anything too stressful on the bike such as drops, jumps or fast descents.
The tyres are fine for any offroading the bike can handle. I use 700cx28mm touring tyres for offroad riding.
Your adjustable stem is a potential weak point. For stability, place it in a lower position then make sure it it completely tight.